What MacBook model do you have (System Profiler will help you find out)?
That "eject" sound is the optical drive checking itself and looking for any media which might be inserted into it.
Normally that should only occur during start up or waking from sleep (there is a firmware fix for this), but not during use, unless you slap the MB a bit harder on the side of the SuperDrive - my iBook worked that way.
Since when does that sound occur?